This invention relates to optical time domain reflectometers (OTDRs), and specifically to the averaging means used to extract, from a high noise level, the signals of interest to the OTDR.
Repetitive firings of a laser from an OTDR into one end of a fiber optic strand cause a return to the OTDR of analog signals representing the reflection characteristics along the length of the strand. The waveform displaying these reflection characteristics is corrupted by noise. The waveform must be recovered to some arbitrary degree of accuracy. The typical means by which this is done is digitizing the waveform, by sampling it periodically and co-adding the results. This technique improves the signal-to-noise ratio by the logarithm of the number of waveforms co-added. In the technique used in present OTDRs, the waveform is digitized with a "flash" converter. Then from very fast static RAM's, the accumulated value of the point being measured is recalled. The latest point is added to this data, and placed back in memory. The next point on the waveform is digitized, and so on.
The problem with this method is that there is a limit to how finely the waveform can be digitized in time. It takes about 80 nanoseconds (nS) to digitize the signal, change the RAM address, wait for the addition to occur, and then place the result back in memory. To get finer resolution it is necessary to first average a set of data with 80 nS data spacing, and then run a new set of data with the sampling process offset in time by the desired amount. For example, 16 sets of averages must be performed to get a desired 5 nS time resolution. Needless to say, this approach is cumbersome and time consuming, and requires a good deal of circuitry to implement. The fast digital ICs required also consume a great deal of power.